During the last few years, cellular network operators have started to offer mobile broadband based on Wideband Code Division Multiple Access (VVCDMA)/High Speed Packet Access (HSPA). Fuelled by new mobile devices designed for data applications, the end user performance requirements have been steadily increasing. The large uptake of mobile broadband has resulted in significant growth in the traffic volumes that need to be handled by the HSPA networks. Therefore, techniques that allow cellular network operators to manage their network resources more efficiently are becoming increasingly important.
Standardised by the third Generation Partnership Project (3GPP), High Speed Packet Access (HSPA) supports the provision of voice services in combination with mobile broadband data services. HSPA comprises High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA) and HSPA+. HSDPA allows networks based on the Universal Mobile Telecommunications System (UMTS) to have higher data transfer speeds and capacity. In HSDPA, a new transport layer channel, High Speed Downlink Shared Channel (HS-DSCH), has been added to the UMTS release 5 and further specification. It is implemented by introducing three new physical layer channels: High Speed-Shared Control Channel (HS-SCCH), Uplink High Speed-Dedicated Physical Control Channel (HS-DPCCH) and High Speed-Physical Downlink Shared Channel (HS-PDSCH). The HS-SCCH informs the user equipment/mobile device that data will be sent on the HS-DSCH, 2 slots ahead. The HS-DPCCH carries acknowledgment information and current Channel Quality Indicator (CQI) of the user equipment. This value is then used by the base station to calculate how much data to send to the user equipments on the next or a future transmission. The HS-PDSCH is the channel mapped to the above HS-DSCH transport channel that carries actual user data.
Some techniques that can be used to improve the downlink performance for end users include 4-branch MIMO (multiple input, multiple output), multiflow communication, multi carrier deployment, etc. Since improvements in spectral efficiency per link are approaching theoretical limits, the next generation technology is about improving the spectral efficiency per unit area. In other words, the additional features for HSDPA need to provide a uniform user experience to users anywhere inside a cell by changing the topology of traditional networks. Currently 3GPP has been working on this aspect through studies of heterogeneous network structures (see for example RP-121436 “Study on UMTS Heterogeneous Networks”, R1-124512 “Initial considerations on Heterogeneous Networks for UMTS” and R1-124513 “Heterogeneous Network Deployment Scenarios”.
Traditionally, networks are arranged in a homogeneous structure, with the network comprising base stations (also known as Node Bs) arranged in a planned layout in which all base stations have similar transmit power levels, antenna patterns, receiver noise floors, and similar backhaul connectivity to the data network. Moreover, all base stations offer unrestricted access to consumer mobile devices (also known as User Equipments—UEs) in the network, and serve roughly the same number of mobile devices. Current wireless systems falling under this category include, for example, Global System for Mobile communications (GSM), WCDMA, HSPA, Long Term Evolution (LTE) and Worldwide Interoperability for Microwave Access (WiMAX).
More recently, heterogeneous mobile communications network structures have been considered. Heterogeneous networks are an efficient network deployment solution for satisfying the ever-increasing demand of mobile broadband services. In a heterogeneous network, a low- or lower-power node (LPN), for example a picocell, microcell or femtocell base station (NodeB), is placed in a traffic hot spot or coverage hole within the coverage area of a high- or higher-power node, for example a macrocell base station, to better serve nearby mobile devices. Deploying a low power node in a traffic hot spot may significantly reduce the load in the macro or other higher-power cell covering the area. The power at which the picocell, microcell or femtocell base stations transmit can be of the order of 2 Watts (W), which compares to around 40 W for a macrocell base station.
FIG. 1 shows an exemplary heterogeneous UMTS mobile communication network 2 that comprises a macrocell node/base station (NodeB) 4 that establishes a cell with a coverage area 6. Two low power nodes/base stations 8, 10 (for example femtocell base stations) are located within the coverage area 6 of the macrocell base station 4, each defining a respective coverage area 12, 14.
The base stations 4, 8, 10 are connected to a network node 16, such as a radio network controller, RNC, 16 which controls the base stations 4, 8, 10 and manages radio resources and mobility in the cell. The RNC 16 may connect directly to the macrocell base station 4, and connect via the Internet to the low power base stations 8, 10. The RNC 16 also connects the base stations 4, 8, 10 to higher parts of the network 2, such as the core network, CN (not shown in FIG. 1).
A mobile device (UE) 18 is shown in the coverage area 12 of low power base station 8 and the coverage area 6 of the macrocell base station 4.
In some heterogeneous network deployments, each of the cells defined by the macrocell base station 4 and low power base stations 8, 10 have respective cell identifiers, which means that the macrocell base station 4 and the low power base stations 8, 10 effectively define different cells. Simulations show that using low power base stations 8, 10 in a macrocell base station coverage area 6 in this way offers load balancing, which results in large gains in system throughput as well as cell edge user throughput. However, a disadvantage with this arrangement is that as each low power base station creates its own cell, it is necessary for the mobile device 18 to perform a soft handover from one low power base station 8, 10 to the macrocell base station 4 or to another of the low power base stations 8, 10, which means higher layer signalling is required to perform the handover.
However, in other heterogeneous network deployments, each of the low power base stations 8, 10 use the same (i.e. a shared) cell identifier as the macrocell base station 4, which means that the macrocell base station 4 and the low power base stations 8, 10 are part of the same cell and effectively ‘assist’ the macrocell base station 4 in providing service to the mobile device 18. This type of deployment is known as a shared or combined cell, and is generally illustrated in FIG. 2. In a combined cell, all the nodes (i.e. macrocell base station 4 and low power base stations 8, 10 are connected via high speed links 19, such as optical links. A central scheduler or controller (not shown) is connected to the RNC 16 and also to any one of the base stations (usually to the macrocell base station) and takes responsibility for collecting operational statistics information from network environment measurements. This type of deployment avoids the need for the mobile device 18 to perform frequent soft handovers, and thus avoids the need for additional higher layer signalling.
As described in “Heterogeneous Network Deployment Scenarios” (R1-124513), in a shared or combined cell deployment of low power base stations having the same cell identifier as the overlying macrocell base station (and a deployment where the low power base stations and the macrocell base station use the same frequency), it is possible for a mobile device to only receive data from one or more antennas of a single base station (e.g. macrocell base station 4 or low power base station 8 for mobile device 18, but not from both), or to operate such that it receives data from one or more antennas of both the macrocell base station and one or more low power base stations simultaneously (e.g. from both macrocell base station 4 and low power base station 8 for mobile device 18). This latter arrangement can be considered as a distributed MIMO (multiple-input, multiple-output) arrangement. The decision of which nodes to use to transmit data to a specific UE is made by the central scheduler/controller based on information provided by the UE or based on information from other sources.
Based on the possibility for there to be data transmission from different nodes, transmission modes in a combined cell deployment can be divided into:                a. Single Frequency Network (SFN): In this mode all nodes transmit the same pilot channel, and data and control information is transmitted from all the nodes. Note that in this case only one UE can be served from all the nodes at any instant in time. Hence this mode is useful for coverage improvement. Furthermore, this mode works for all legacy UEs (i.e. UEs not complying with the most recent releases of the standards). FIG. 3 provides an illustration of the transmissions of various channels in a SFN combined cell deployment. Thus, it can be seen that the HSDPA pilot channel (which is called the Primary Common Pilot Channel (P-CPICH) and is used for estimating the channel for computing the channel quality indicator (CQI) of the mobile device) and the same HS-SCCH and HS-PDSCH are transmitted by each of the nodes in the combined cell deployment (so a macrocell node and three low power nodes).        b. Node Selection with Spatial Re-use: In this mode, even though all the nodes transmit the same pilot channel; data and the control information transmitted from one node is different from that from every other (or at least one other node), i.e. one or more nodes will be serving a specific UE, while at the same time different data and control channel information will be sent to a different UE. Hence the spatial resources can be reused. This mode provides load balancing gains, which means the capacity of the combined cell can be increased significantly. FIG. 4 provides an illustration of the transmission of various channels in a spatial reuse combined cell deployment (the D-CPICH shown in FIG. 4 is a demodulation/Dedicated pilot channel used for estimating the channel for data demodulation). Thus, it can be seen that each of the nodes (which are not labelled as specific nodes but include a macrocell node and three low power nodes) each transmit the same P-CPICH, but each of the nodes transmit respective D-CPICH, HS-SCCH and HS-PDSCH channels (where a common shading pattern indicates that the node(s) is(are) transmitting to the same UE).        